Electrowetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by the application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid. Nanofluid. (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 10, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of array element electrodes 12 (e.g., 12A and 12B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 12. A liquid droplet 14, including a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate 10 and a top substrate 16. A suitable gap or channel between the two substrates may be realized by means of a spacer 18, and a non-polar surround fluid 20 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 14. An insulator layer 22 disposed upon the lower substrate 10 separates the conductive element electrodes 12A, 12B from a first hydrophobic coating 24 upon which the liquid droplet 14 sits with a contact angle 26 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).
On the top substrate 16 is a second hydrophobic coating 28 with which the liquid droplet 14 may come into contact. Interposed between the top substrate 16 and the second hydrophobic coating 28 is a reference electrode 30.
The contact angle θ is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-to liquid (γSL), the liquid-to non-polar surrounding fluid (γLG) and the solid to non-polar surrounding fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. reference electrode 30, element electrodes 12, 12A and 12B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 24. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 12A and 12B), the liquid droplet 14 may be moved in the lateral plane between the two substrates 10 and 16.
Example configurations and operation of EWOD devices are described in the following. U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions. U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials. U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 10.        TFT-based thin film electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require electro-wetting voltages in excess of 20V to be applied.        
As described above with respect to the representative EWOD structure, the EWOD channel or gap defined by the two substrates initially is filled with the non-polar fluid (oil). The liquid droplets 14 including a polar material, i.e., the droplets to be manipulated by operation of the EWOD device, must be inputted from a fluid reservoir into the EWOD channel or gap. As the fluid from the reservoir for the droplets in inputted, oil gets displaced and is removed from the EWOD channel.
Different mechanisms have been devised for the inputting or loading of polar fluid for the liquid droplets into such devices. For example, U.S. Pat. No. 8,702,938 (Srinivasan et al., issued Apr. 22, 2014) describes an EWOD device with a plastic frame surrounding a glass substrate. The plastic frame contains holes which provide a fluid path from the exterior of a droplet actuator and into the EWOD channel or gap. US20100282609 (Pollack et al., published Nov. 11, 2010) describes the use of a piston mechanism to force fluid into reservoirs contained in a device already loaded with oil. US20100032293 (Pollack et al., published Feb. 11, 2010) describes a filling mechanism where fluid flows onto the array containing oil via a variable pressure source, and droplets of fluid are held on the array and the excess fluid is drained. US20130161193 (Jacobs et al., published Jun. 27, 2013) describes a method to drive fluid into a device filled with oil by using, for example, a bistable actuator. Mechanisms for precise control of the input of fluid into the EWOD channel or gap are not taught in these patent documents.
U.S. Pat. No. 9,630,180 (Srinivasan et al., issued Apr. 25, 2017) describes controlling the hydrostatic head of a droplet actuator to avoid fluid spontaneously flowing into the gap between the bottom and top substrates. The patent illustrates schematics of potential droplet actuators and associated electrode patterns for drawing fluid out of such a droplet actuator. In addition, this patent describes having a waste reservoir which is made as large as possible to lower the pressure at the reservoir. As representative, the teachings include restrictions on the relative sizes of different parts of the droplet actuator.
WO 2017/047082 (Walton et al., published Mar. 23, 2017) describes partially filling a device with a metered volume of oil. Fluid is then input into the device easily by venting air. An exemplary embodiment includes filling the device with oil, adding fluid to the input ports, then extracting a sufficient volume of oil to enable at least some of the fluid to enter the device. With the disclosed device configurations, there is a limit to the quantity of fluid that can be processed by such a device, and the loading of fluid occurs with the user manually removing a portion of the oil.